Glucose isomerase is an enzyme that can be employed to catalyze the conversion of glucose (dextrose) to fructose (levulose). It is known that glucose isomerase can be produced by fermentation of certain organisms, such as Streptomyces flavovirens, Streptomyces echinatur, Streptomyces achromogenus, Streptomyces albus, Streptomyces olivaceus, Bacillus coagulans and the like, in appropriate nutrient media. The glucose isomerase is formed inside the bacterial cells which grow during its production. The cells can be filtered off from the fermentation beer and used directly as a source of glucose isomerase. Direct commercial use of such enzyme-containing bacterial cells had been hampered, however, by a major disadvantage. The enzyme activity was lost from the cells during use and thus the useful life of the cells was reduced. This disadvantage was overcome by the treatment of the bacterial cells with glutaraldehyde as described in U.S. Pat. No. 3,779,869. Additional techniques for immobilizing the enzyme activity in bacterial cells as well as for forming aggregates of such enzyme-containing bacterial cells are described for example, in U.S. Pat. No. 3,821,086 and its U.S. Pat. Re. Nos. 29,130 and 29,136 and in South African Pat. No. 73/5917. The above U.S. patents relate to use of certain anionic and cationic polyelectrolyte flocculating agents. The South African patent discloses various combinations of binders, reinforcing agents and cross-linking agents. While the above techniques provided bacterial cell aggregates which generally retained their enzyme activity during use, there was still a need to increase the hardness of the aggregates so that they could be commercially used in reactor beds of increasing depth. U.S. Pat. No. 3,935,069 describes the addition of certain metallic compounds in conjunction with polyelectrolyte flocculating agents to improve the hardness. However, this technique has limited utility.